1. Technical Field
This invention relates to a freeze dryer apparatus and more particularly to a freeze dryer apparatus having an interim condensing system. The interim condensing system condenses vapors within the product chamber of the freeze dryer apparatus when the main condenser, which normally condenses the vapors from within the product chamber, needs to be defrosted.
2. Description of the Prior Art
Freeze drying, called sublimation or lyophilization, is a dehydration process accomplished under precisely controlled conditions. Freeze drying causes the ice within a specimen to change from a solid directly into a gaseous/vapor state, bypassing the liquid state altogether. Unlike specimens dried from the usual non-frozen condition, freeze-dried specimens do not distort nor shrink. Applied to the taxidermy field, the result is a truly lifelike specimen, which is the goal of all taxidermists and museum curators.
Sublimation begins at the outer surface of the specimen and recedes towards the center of the specimen as drying advances. As ice molecules change to a vapor state, they are carried away from the specimen by lower pressure elsewhere. This lower pressure region is the ice condenser. Most specimens are dried by using a surrounding product chamber temperature of -5.degree. F. (about -20.degree. C.). The refrigerated condenser, typically running at -60.degree. F. (about -51.degree. C.), presents substantially lower vapor pressure which causes a migration of the vapor from the -5.degree. F. (about -20.degree. C.) product chamber to the -60.degree. F. (about -51.degree. C.) condenser region. A vacuum system is also provided which removes almost all of the air from the product chamber and condenser, allowing the ice/vapor molecules to move unhampered to the condenser.
Typically, freeze dryer apparatuses have an external condenser. After a period of use, the continuous condensation of the vapors at the condenser causes ice to build up on the condenser. Therefore, the condenser must be defrosted, drained, and recooled in order to continue to function properly. This process, typically taking ten (10) to thirty (30) minutes, results in a problem in that the product chamber is closed off to any form of condensing during the defrost period. During this off period in freeze driers, the pressure in the product chamber invariably rises due to water vapor still being evolved from the frozen samples with no condensing capability being available.
This pressure rise in turn causes the sample temperature to rise, possibly resulting in melting of the samples and thus a partial or complete loss of freeze drying benefits. Secondarily, when the defrosted condenser is finally returned to duty, it is subjected to a very high loading due to the accumulated vapor and higher sample temperatures.
Attempts have been made to solve this problem by creating specimen dryers with two (2) or more equal sized external condensers, with either a common or independent refrigeration system. These systems are capable of continuous freeze drying. However, these systems are expensive in terms of the substantial cost of this second external condenser.
Steinkamp, U.S. Pat. No. 4,949,473, issued Aug. 21, 1990, discloses a freeze drying apparatus which includes two condensers. The second condenser serves as a fail-safe condensation means in the event of an apparatus malfunction in the primary condenser.
Northstar (Nisswa, MN) markets portable freeze dryer systems. One, the model L-48104 processor, has twin vacuum pumps and dual condensers. The dual condensers are both full size external condensers, and each may be connected to its own refrigeration system. The freeze dryer system includes isolation valves which allow for a defrost cycle at any time without interrupting the drying process, thus allowing for continuous operation.
However, the problems discussed above in regard to the advantages of eliminating two equal-sized external condensers are applicable to this Northstar system.
It is thus an object of the subject invention to provide a freeze dryer apparatus which provides continuous freeze drying without the need of a second external condenser to replace the main condenser while it is being thawed.